From the past, addition curable silicone resin compositions are used as encapsulant for semiconductor chips such as LED because of fast cure and heat resistance and light resistance of cured products. For example, Patent Document 1 describes an addition curable silicone resin composition which exhibits a high bond strength to LED packages of thermoplastic resins, typically PPA. Patent Document 2 describes the encapsulation of an optoelectronic chip with an addition curable silicone resin composition by compression molding.
Although addition curable silicone resin compositions are widely used as semiconductor encapsulant, their properties are still unsatisfactory. Particularly in the semiconductor encapsulant field where the encapsulating resin is stressed due to a temperature difference between the ambient environment and during current conduction, materials having crack resistance are required. Because of poor crack resistance, silicone resins are susceptible to cracking. To solve the problem, gel or rubber-like soft silicone resins are used. When semiconductor chips are encapsulated by compression molding using a compression mold or transfer mold, the gel or rubber-like silicone resins are inadequate because they are so tacky, giving rise to a stick-in-mold problem. There is a need for silicone resins which have a sufficient hardness for mold release and are yet resistant to stresses.
For imparting toughness to a cured silicone resin while maintaining hardness, an attempt to incorporate a silphenylene skeleton into a silicone resin is made in Patent Documents 3 and 4. As compared with the general method of establishing a high hardness by increasing the crosslinking density of silicone resin, this method establishes a high hardness by incorporating a silphenylene skeleton into a silicone resin to restrain motion of the polymer chain. Thus the resin has rigidity and hardness. While the method of increasing the crosslinking density generally makes the resin brittle, the method of incorporating a silphenylene skeleton is advantageous in that the resin exerts a toughness without embrittlement because the silphenylene skeleton is linear. However, the method of incorporating a silphenylene monomer into a silicone resin skeleton as described in Patent Documents 3 and 4, achieves weak binding of the molecular chain and thus fails to form a resin having a high hardness and yet satisfactory stress resistance.